토양에서 228Ac이 40K의 비방사능 평가에 미치는 영향

Abstract

The Effect of 228Ac on the Specific Activity Evaluation of 40K in the soil Eun Seok Shin

Department of Physics, The Graduate School, Pukyong National University

Abstract The safe regulation for natural radiation raised to issue in Korea as well as the other countries by starting of the law about radiation in life. So, it was important to measure the activities of natural radiation exactly. Specially, the activity of 40K was higher than U-series, Th-series and Ac-series etc. in soil. But the specific activity of 40K has some error because its gamma ray energy is 1460.7keV and also one of gamma ray energies for 228Ac is 1459.14 keV that is overlaped with the energy line of 40K. In this study, we took 117 soil samples at 38 positions in 3 areas, and analysed them in the laboratory by gamma spectroscopy using the HPGe detectors. The samples were dried in 105 ℃ during 24 hours and selected homogeneous particles of the using the standard mesh sized 200. The selected soil was filled to 450 ml Marinelli beakers and measured during 50,000 seconds per a sample. The efficiencies and energy calibrations of the HPGe detectors were performed using the certificated standard sources that had made in KRISS. We determined the specific activities for 40K and 232Th series(228Ac) of the samples, found the corelations 40K between 228Ac from errors considered with efficiencies and branching ratio of each energy line, and compared with other studies. We had the results as below ; First, the specific activities of 232Th series(228Ac) were merely measured to 21.34∼58.13 Bq/kg-dry. Second, the contributed ratio of 228Ac for 40K were weak as 0.31∼1.18%, and the error of 40K was 1% when the ratio of 40K/228Ac was about 7.5. Finally, its relation equation is ΔKac(%) = 7.49596×(228Ac/40K) － 0.00156. We should try to measure the specific activity as more accurately as possible about natural radiation so that make a estimation of radiation exposure from it. This study is helpful to reduce error in the activity of 40K for scientists majoring in environmental radiation.